Pressure transducer employing on-chip resistor compensation

ABSTRACT

A dielectrically isolated temperature compensated pressure transducer including: a wafer including a deflectable diaphragm formed therein, the diaphragm being capable of deflecting in response to an applied pressure, and the diaphragm defining an active region surrounded by an inactive region of the wafer; a plurality of dielectrically isolated piezoresistive elements formed on the active region of the wafer and coupled together to form a Wheatstone bridge configuration so as to cooperatively provide an output signal in response to and indicative of an amount of deflection of the diaphragm, the plurality of piezoresistive elements being undesirably operative to introduce an undesirable error into the output according to exposure of the wafer to an environmental condition; and, a dielectrically isolated resistor formed on the inactive region of the wafer and electrically coupled in series to the plurality of piezoresistive elements so as to at least partially compensate for the undesirable error.

FIELD OF INVENTION

[0001] The present invention relates to piezoresistive pressure sensorsand more particularly temperature compensated pressure transducers.

BACKGROUND OF INVENTION

[0002] It is well known in a piezoresistive Wheatstone bridge havingfour equal piezoresistors of resistance R_(B) of which two increase withpositive strain and two decrease with an equal negative strain, that thechange of voltage ΔV across the bridge is given by$\frac{\Delta \quad V}{V_{B}} = \frac{\Delta \quad R_{B}}{R_{B}}$

[0003] and that$\frac{\Delta \quad R}{R} = {\varepsilon \quad {GF}}$

[0004] where ε is the strain, V_(B) is the voltage applied across thebridge and GF is the gauge factor. It is also well known that the gaugefactor decreases as a function of bridge temperature. The rate of changeof gauge factor with temperature is usually referred to as TCGF ortemperature coefficient of gauge factor. Thus, for a constant voltageapplied across the bridge, the output will decrease as a function oftemperature. It is also well known that resistance of the bridgeelements increase as a function of temperature. The change of resistancewith temperature is referred to as TCR or temperature coefficient ofresistance. For highly doped P-type silicon, the TCGF is approximately−2%/100° F. to −3%/100° F. while the TCR is approximately +10%/100° F.Referring now to FIG. 1, one way to make the output voltage moreindependent of temperature using a constant voltage source 15 is toplace a temperature independent resistor R_(S) in series with the bridge2. Thus, as temperature increases the bridge resistance increases andmore of the supply voltage appears across the bridge. For this case thebridge voltage VB is given by$V_{B} = {\frac{R_{B}}{R_{B} + R_{S}}V_{0}}$

[0005] Thus, by appropriate choice of the ratio of R_(S) to R_(B), thedesired increase of bridge voltage with temperature can be obtained.This compensation scheme can be used in many applications. An example ofsuch an apparatus and method is illustrated in U.S. Pat. No. 3,245,252,entitled “TEMPERATURE COMPENSATED SEMICONDUCTOR STRAIN GAGE UNIT” issuedApr. 12, 1966, the entire disclosure of which is hereby incorporated byreference as if being set forth herein in it entirety. However, thereare some instances when such an approach is unsuitable for certainneeds. An example of such an application is where certain extremelytight specifications are needed, then the passive resistor alone can notaccomplish the desired effect. This is especially true when the pressuretransducer must be extremely precise at either the extreme cold end orextreme hot end of the operating temperature range of the device.

[0006] It is an object of the present invention to provide an improvedtemperature compensated transducer suitable for use in these types ofapplications.

SUMMARY OF INVENTION

[0007] A dielectrically isolated temperature compensated pressuretransducer including: a wafer including a deflectable diaphragm formedtherein, the diaphragm being capable of deflecting in response to anapplied pressure, and the diaphragm defining an active region surroundedby an inactive region of the wafer; a plurality of dielectricallyisolated piezoresistive elements formed on the active region of thewafer and coupled together to form a Wheatstone bridge configuration soas to cooperatively provide an output signal in response to andindicative of an amount of deflection of the diaphragm, the plurality ofpiezoresistive elements being undesirably operative to introduce anundesirable error into the output according to exposure of the wafer toan environmental condition; and, a dielectrically isolated resistorformed on the inactive region of the wafer and electrically coupled inseries to the plurality of piezoresistive elements so as to at leastpartially compensate for the undesirable error.

BRIEF DESCRIPTION OF THE FIGURES

[0008]FIG. 1 illustrates a conventional temperature compensationincluding an output circuit which uses a single passive resistor inseries with the piezoresistive bridge.

[0009]FIG. 2 illustrates a plan view of a dielectrically isolated sensorstructure according to the present invention.

[0010]FIG. 3 illustrates an electrical representation of the presentinvention.

[0011]FIG. 4 illustrates the potential output characteristics of apressure transducer versus temperature according to the presentinvention, depending upon the magnitude of the on-chip resistor.

[0012]FIG. 5 illustrates the advantage gained utilizing the invention,compared to the standard methodology.

DETAILED DESCRIPTION OF THE INVENTION

[0013] According to the present invention, in order to fit thesespecifications it is necessary to adjust slightly the temperaturevariation of the bridge voltage such that it is easier to compensate thetransducer.

[0014] In FIG. 1 is shown a conventional temperature compensationcircuit using a single, passive resistor R_(S) in series with thepiezoresistive Wheatstone bridge 2. According to the present invention,a resistor which has a substantially same temperature coefficient as thebridge elements, hereafter called the “on-chip resistor”, is placed nearan edge of the sensor wafer on the inactive portion of the sensor. Moreparticularly, the TCR of the on-chip resistor is preferablysubstantially identical to that of each of the arms of the Wheatstonebridge 2. The on-chip resistor is shunted with a passive resistiveelement which is largely temperature independent. The on-chip resistoris placed in a region of minimum stress on the sensor so that it doesnot change resistance with pressure. Instead, it is intended to changesolely with temperature in the same fashion that the otherpiezoresistive elements of the Wheatstone bridge 2 change. By shuntingthe on-chip resistor with a temperature independent fixed resistorhaving a TCR of substantially zero, the TCR of the parallel combinationcan be adjusted depending upon the magnitude of the on-chip and shuntresistors. This is because a parallel combination of resistors withdifferent TCRs will have a different combined TCR then each individualone. By varying the magnitudes of the on-chip and shunt resistors, awide range of TCR resistances in series with the basic Wheatstone bridge2 can be obtained. Thus, by adjusting the size of the on-chip and shuntresistor it is possible to produce a wide range of different adjustmentsto the increase of bridge voltage with temperature. The equation belowshows how the output voltage is effected by all of the resistors:$\frac{V_{out}}{V_{in}} = {( \frac{V_{out}}{V_{in}} )_{OLD}\frac{R_{bridge}( {R_{shunt} + R_{OnChip}} )}{\begin{matrix}{{R_{shunt}R_{bridge}} + {R_{OnChip}R_{bridge}} +} \\{{R_{span}R_{shunt}} + {R_{span}R_{OnChip}} + {R_{shunt}R_{OnChip}}}\end{matrix}}}$

[0015] It can be seen from this equation that by making R_(OnChip) thesame order of magnitude as the bridge resistance and by making R_(shunt)much larger it is possible to make the term shown have a much differentTCR then a bridge would have alone. If a greater effect is desired thena larger R_(OnChip) can be used.$( \frac{V_{{out}^{\prime}}}{V_{in}} )_{OLD}$

[0016] being the pre-temperature compensated tranducer's sensitivity.

[0017] Referring now to FIGS. 2-5, like references identify likeelements of the invention. FIG. 2 illustrates a plan view of adielectrically isolated bridge sensor circuit structure 10 formed on asilicon wafer according to the present invention. The structure 10 canbe preferably formed in accordance with the teachings commonly assignedU.S. Pat. No. 5,286,671, entitled, “Fusion bonding technique for use infabricating semiconductor devices” the entire disclosure of which isalso incorporated by reference as if being set forth in its entiretyherein. The structure 10 includes in the preferred embodiment, adeflectable diaphragm 20 having piezoresistors 30, 40, 50 and 60electrically coupled in a Wheatstone Bridge configuration formed on ortherein. Piezoresistors 30, 60 each decrease with positive normal stressand piezoresistors 40, 50 each increase with positive normal stress inresponse to deflection of the diaphragm 20 as is well known. Thepiezoresistors 30, 40, 50, 60 are preferably formed of highly dopedP+silicon. It is understood that a number of such sensors can be made atthe same time on a large substrate. The circuit nodes of the Wheatstonebridge include four oversized P+ silicon electrical contact areas orfingers 70, 70′, 80, 90, 100 which are mainly located in non-activeareas of the wafer 5. It should be understood the active portions of thewafer can be defined as that portion defined by the diaphragm 20, asthis portion deflects in response to an applied pressure as is wellknown. The remaining portions are referred to as the non-active regions.The term “finger” is used to indicate those areas 70, 70′, 80, 90, 100which project from the piezoresistors 30, 40, 50, 60. The areas 70, 70′,80, 90, 100 are further adapted to be used as bonding pads toelectrically couple to the structure 10. The structure 10 furtherincludes on-chip resistor 110 electrically coupled between bonding pad100 and bonding pad 120. Bonding pad 70′ can be shorted to pad 70 toform a full bridge configuration using the piezoresistos 30, 40, 50, 60.

[0018] The wafer 100 is preferably fabricated using the method disclosedin commonly assigned U.S. Pat. No. 5,286,671 entitled “DiffusionEnhanced Fusion Bonding”, the entire disclosure of which is also herebyincorporated by reference as if being set forth herein in its entirety.Alternatively, any conventional wafer processing technique which enablesdielectically isolated piezoresistive sensor elements 30, 40, 50, 60 tobe formed on semiconductor material using dielectric films of SiO₂ orthe like could be used.

[0019] Referring now also to FIG. 3, therein is illustrated anelectrical equivalence of the sensor circuit structure 10 incorporatedinto a sensor system 12. The system 12 includes the sensor circuitstructure 10, an off-chip shunt resistor 130 coupled in parallel acrosson-chip resistor 110 and off-chip span resistor 140 coupled in serieswith the structure 10. Basically, the structure 10 is electricallycoupled to an excitation voltage V_(in) through serially coupledresistor 140 using pads 120 and 80. An output voltage V_(out) ismeasured using the pads 70 and 90.

[0020] Referring now also to FIG. 4, one can easily ascertain that byadjusting the value or resistance of on-chip resistor 110, it ispossible to shift either end of the temperature range over which thedevice is operable up or down. This shift sacrifices the other end ofthe range, however this is often time acceptable because the sameprecision is not required at both ends of the temperature range. It isalso easy to ascertain that by changing the ratio of the on-chipresistor 110 to that of the shunting resistor 130, similar adjustmentsmay be made.

[0021] Referring now also to FIG. 5, it illustrates that whileconventional passive resistor compensation techniques can give valuesnear zero percent for a temperature range of −40° F. to 200° F., whenthe temperature to which the transducer is exposed rises above this, themethod and device according to the present invention can beadvantageously used. Further, the present method and device enables foroverall smoothing of the performance curve of the Wheatstone Bridgeoutput V^(in) as is clearly illustrated in FIG. 5.

[0022] This new on chip compensation scheme is not meant to replace alltraditional compensation schemes, instead it is meant to enhance some incertain circumstances. Namely when there is a very tight specificationat either end of the temperature range.

[0023] Although the invention has been described and pictured in apreferred form with a certain degree of particularity, it is understoodthat the present disclosure of the preferred form, has been made only byway of example, and that numerous changes in the details of constructionand combination and arrangement of parts may be made without departingfrom the spirit and scope of the invention as hereinafter claimed. It isintended that the patent shall cover by suitable expression in theappended claim, whatever features of patentable novelty exist in theinvention disclosed.

We claim:
 1. A dielectrically isolated temperature compensated pressuretransducer comprising: a wafer including a deflectable diaphragm formedtherein, said diaphragm being capable of deflecting in response to anapplied pressure, and said diaphragm defining an active regionsurrounded by an inactive region of said wafer; a plurality ofdielectrically isolated piezoresistive elements formed on said activeregion of said wafer and coupled together to form a Wheatstone bridgeconfiguration so as to cooperatively provide an output signal inresponse to and indicative of an amount of deflection of said diaphragm,said plurality of piezoresistive elements being undesirably operative tointroduce an undesirable error into said output according to exposure ofsaid wafer to an environmental condition; and, a dielectrically isolatedresistor formed on said inactive region of said wafer and electricallycoupled in series to said plurality of piezoresistive elements so as toat least partially compensate for said undesirable error.
 2. Thetransducer of claim 1, further comprising a second resistor coupled inparallel across said resistor.
 3. The transducer of claim 2, whereinsaid second resistor is substantially temperature independent within anoperating temperature range of said transducer.
 4. The transducer ofclaim 2, further comprising a third resistor coupled in series with saidplurality of piezoresistive elements.
 5. The transducer of claim 4,wherein${\frac{V_{out}}{V_{in}} = {( \frac{V_{out}}{V_{in}} )_{OLD}\frac{R_{bridge}( {R_{shunt} + R_{OnChip}} )}{\begin{matrix}{{R_{shunt}R_{bridge}} + {R_{OnChip}R_{bridge}} +} \\{{R_{span}R_{shunt}} + {R_{span}R_{OnChip}} + {R_{shunt}R_{OnChip}}}\end{matrix}}}},$

Vout is an output voltage from said transducer, Vin is an excitationvoltage of said transducer, (V_(out)/V_(in))_(OLD) is the transducer'spre-temperature compensated sensitivity, R_(bridge) is the resultingresistance of said plurality of piezoresistive devices in response tosaid applied pressure, R_(shunt) is the resistance of said secondresistor, R_(OnChip) is the resistance of said resistor and R_(span) isthe resistance of said third resistor.
 6. The transducer of claim 1,wherein each of said plurality of piezoresistors has a substantiallysame temperature coefficient of resistance.
 7. The transducer of claim6, wherein said Wheatstone bridge configuration includes two open arms.8. The transducer of claim 7, wherein a temperature coefficient ofresistance of said resistor is substantially the same as that of each ofsaid arms of Wheatstone bridge.
 9. A temperature compensated pressuretransducer comprising: a wafer including a deflectable diaphragm whichdefines an active region of said wafer surrounded by an inactive regionof said wafer; a plurality of piezoresistors formed on said activeregion of said wafer and coupled in a Wheatstone bridge configuration soas to generate a signal including a first portion indicative of anamount of deflection of said deflectable diaphragm and a second portionindicative of a temperature of said Wheatstone bridge; and, a resistorformed on said inactive region of said wafer and electrically coupled tosaid Wheatstone bridge so as to at least partially cancel said secondportion of said signal.
 10. The transducer of claim 9, furthercomprising a second resistor coupled in parallel across said resistor.11. The device of claim 16, wherein said second resistor issubstantially temperature independent within an operating temperaturerange of said transducer.
 12. The transducer of claim 10, furthercomprising a third resistor coupled in series with said plurality ofpiezoresistive elements.
 13. The transducer of claim 12, wherein${\frac{V_{out}}{V_{in}} = {( \frac{V_{out}}{V_{in}} )_{OLD}\frac{R_{bridge}( {R_{shunt} + R_{OnChip}} )}{\begin{matrix}{{R_{shunt}R_{bridge}} + {R_{OnChip}R_{bridge}} +} \\{{R_{span}R_{shunt}} + {R_{span}R_{OnChip}} + {R_{shunt}R_{OnChip}}}\end{matrix}}}},$

Vout is an output voltage from said transducer, Vin is an excitationvoltage of said transducer, (V_(out)/V_(in))_(OLD) is the transducerspre-sensitivity temperature compensated, R_(bridge) is the resultingresistance of said plurality of piezoresistive devices in response tosaid applied pressure, R_(shunt) is the resistance of said secondresistor, R_(OnChip) is the resistance of said resistor and R_(span) isthe resistance of said third resistor.
 14. The transducer of claim 13,wherein each of said plurality of piezoresistors has a substantiallysame temperature coefficient of resistance.
 15. The transducer of claim14, wherein said Wheatstone bridge configuration includes two open arms.16. The transducer of claim 14, wherein a temperature coefficient ofresistance of said resistor is substantially the same as that of each ofsaid arms of Wheatstone bridge.
 17. A method for providing a temperaturecompensated pressure transducer comprising: providing a wafer having adeflectable diaphragm formed therein defining an active region whichdeflects in response to an applied pressure and an inactive region;forming a plurality of piezoresistive devices in a Wheatstone bridgeconfiguration on said active region of said wafer such that saidplurality of piezoresistive devices collectively provide an outputindicative of said applied pressure and introduce an error into saidoutput due to an environmental condition to which said plurality ofpiezoresitive devices are exposed; forming a first resistive device onsaid inactive region of said wafer in sufficient proximity to saidpiezoresistive devices such that it will also be exposed to saidenvironmental condition; and, electrically coupling said first resistivedevice to said Wheatstone bridge configuration of piezoresistive devicesso as to at least partially compensate for said error.
 18. The method ofclaim 17, further comprising coupling a second resistive device inparallel to said first resistive device, wherein a resistance of saidfirst resistive device is partially temperature dependent and aresistance of said second device is substantially temperatureindependent.
 19. A dielectrically isolated temperature compensatedpressure transducer comprising: a wafer including a deflectablediaphragm formed therein, said diaphragm being adapted to deflect inresponse to an applied pressure and defining an active region and aninactive region of said wafer; a plurality of dielectrically isolatedpiezoresistive elements formed on said active region of said wafer andelectrically coupled together in a Wheatstone Bridge so as tocooperatively provide an output in response to and indicative of anamount of deflection of said diaphragm, said plurality of piezoresistiveelements being predisposed to introduce an error into said outputresponsively to exposure to an environmental condition; and, adielectrically isolated resistor formed on said inactive region of saidwafer such that it has the same temperature coefficient of resistance asthe piezoresistors but its resistance is independent of the deflectionof the diaphragm; wherein said non-deflection sensitive resistor furtherhaving a second resistor coupled in parallel across said first resistor;wherein said second resistor is substantially temperature independentwithin an operating temperature range of said transducer; a thirdtemperature insensitive resistor coupled in series with said Wheatstoneand said parallel combination of said first and second resistor; thevalues of said first resistor, said second resistor and said thirdresistor relative to the resistance of the Wheatstone Bridgepiezoresistors, chosen such that the voltage across the WheatstoneBridge will change in an optimum way with respect to changes intemperature.
 20. A temperature compensated pressure transducercomprising: a dielectrically isolated sensor including a deflectablediaphragm which defines an active region and an inactive region; aplurality of piezoresistors formed in a Wheatstone bridge configurationon said active region of said sensor so as to generate a signalindicative of an amount of deflection of said deflectable diaphragm and;an on-chip resistor formed on said inactive region of said sensor andelectrically coupled in series with said Wheatstone bridge, said on-chipresistor having essentially the same temperature coefficient ofresistance as the piezoresistors; a second resistor coupled in parallelacross said on-chip first resistor; wherein said second resistor issubstantially temperature independent within an operating temperaturerange of said transducer; and a third resistor coupled in series withsaid plurality of piezoresistive elements and said parallel combinationof said first and second resistors, said resistance values of saidfirst, second and third resistors so chosen as to obtain an optimumvariation with temperature of voltage across the Wheatstone bridge. 21.A method for providing a temperature compensated pressure transducercomprising: providing a dielectrically isolated sensor having adeflectable diaphragm formed therein defining an active region whichdeflects in response to an applied pressure and an inactive region;forming a plurality of piezoresistive devices in a Wheatstone bridgeconfiguration on said active region of said wafer such that saidplurality of piezoresistive devices collectively provide an outputindicative of said applied pressure and introduce an error into saidoutput due to an environmental condition to which said plurality ofpiezoresistive devices are exposed; forming a first resistive device ofappropriate value on said inactive region of said sensor in sufficientproximity to said piezoresistive devices such that it will also beexposed to said environmental condition and have the same TCR as thepiezoresistors; electrically coupling said first resistive device tosaid Wheatstone bridge configuration of piezoresistive devices; couplinga second resistive device in parallel to said first resistive device,wherein the resistance of said second device is substantiallytemperature independent, and chosen to give an appropriate TCR of theparallel combination of said first and second resistor to enhance thetemperature compensation of the Wheatstone bridge, and a third resistorcoupled in series with said plurality of piezoresistive elements andsaid parallel combination of said first and second resistors, saidresistance values of said first, second and third resistors so chosen asto obtain an optimum variation with temperature of voltage across theWheatstone bridge.